Methods and apparatuses for data transmission

ABSTRACT

Methods and devices are disclosed involving crosstalk reduction depending on weighting factors or grouping of transmission channels. In other embodiments, other methods or devices may be used.

RELATED APPLICATIONS

This Application claims priority benefit and is a continuation ofInternational Application PCT/EP2010/050442, which was filed on Jan. 15,2010. The entire contents of the International Application are herebyincorporated herein by reference.

BACKGROUND

So-called vectoring or vectored data transmission is a technique forcoordinated transmission or reception of data from a plurality oftransmitters to a plurality of receivers via a plurality of transmissionchannels in order to improve the transmission, for example to reduce theinfluence of crosstalk. Either transmitters or receivers are co-located.Vectoring is sometimes also referred to as Spectrum Management Level 3.

For example, in DSL (digital subscriber line) transmission systems, forexample VDSL (very high bit rate DSL) transmission systems, data may betransmitted from a central office (CO) or other provider equipment to aplurality of receivers located in different locations, for example incustomer premises (CPE), via a plurality of communication lines.Crosstalk resulting from signals on different lines transmitted in thesame direction, also referred to as far end crosstalk (FEXT), may resultin a reduced data throughput. Through vectoring, signals transmittedover the plurality of communication lines from the central office orreceived via the plurality of communication lines in the central officemay be processed jointly in order to reduce such crosstalk, which jointprocessing corresponds to the above-mentioned vectoring. In thisrespect, the reduction of crosstalk by coordinated transmission ofsignals is sometimes referred to as crosstalk precompensation, whereasthe reduction of crosstalk through joint processing of the receivedsignals is sometimes referred to as crosstalk cancellation. Thecommunication lines which are processed jointly are sometimes referredto as vectored group.

For this kind of crosstalk reduction, for example in an initializationphase of the data transmission system or during operation of the datatransmission system, parameter describing the crosstalk between thecommunication connections are obtained and the crosstalk reduction isperformed based on these parameters.

The computational effort of this crosstalk reduction increases withincreasing number of transmission channels, for example communicationlines, involved. Therefore, in transmission systems involving a largenumber of transmission channels, sometimes so-called partial vectoringis used, where only a part of the transmission channels are subjected tovectoring. In this case, a selection has to be made which transmissionchannels or which crosstalk paths add to the vectored group.

Another possible approach to reduce crosstalk in communication systemsis so-called spectrum balancing, also sometimes referred to as SpectrumManagement Level 2. In this approach, transmission powers for theindividual transmission channels are controlled to reduce the effect ofcrosstalk at least for some transmission channels.

SUMMARY

According to some embodiments, a communication device according to claim1, 9, 10 or 16 or a method according to claim 22 or 28 is provided. Thedependent claims define further embodiments. According to someembodiments of the present invention, a plurality of transmissionchannels is grouped into at least two groups, and a part of transmissionchannels is selected from said plurality of transmission channels forcrosstalk reduction depending on said grouping.

In other embodiments, other features and/or alternative features may beimplemented.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a block diagram illustrating a basic structure of acommunication system according to an embodiment of the presentinvention,

FIG. 2 shows a block diagram illustrating some features of acommunication system according to an embodiment of the presentinvention,

FIG. 3 shows a block diagram illustrating some features of communicationdevices according to some embodiments of the present invention,

FIG. 4 shows a block diagram illustrating some features of communicationdevices according to some embodiments of the present invention.

FIG. 5 shows a flow diagram illustrating a method according to anembodiment of the present invention,

FIGS. 6 to 11 show simulation results for illustrating the effects ofsome embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, some embodiments of the present invention will bedescribed in detail. It is to be understood that the followingdescription is given only for the purpose of illustration and is not tobe taken in a limiting sense. The scope of the invention is not intendedto be limited by the embodiments described hereinafter with reference tothe accompanying drawings, but is to be intended only to be limited bythe appended claims and equivalents thereof.

It is also to be understood that in the following description ofembodiments any direct connection or coupling between functional blocks,devices, components, circuit elements or other physical or functionalunits shown in the drawings or described herein, i.e. any connectionwithout intervening elements could also be implemented by an indirectconnection or coupling, i.e. a connection or coupling with one or moreintervening elements. Furthermore, it should appreciated that functionalblocks or units shown in the drawings may be implemented as separatecircuits in embodiments, but may also be fully or partially implementedin a common circuit in other embodiments. It is further to be understoodthat any connection which is described as being wire-based in thefollowing specification may also be implemented as a wirelesscommunication unless noted to the contrary.

It should be noted that the drawings are provided to give anillustration of some aspects of embodiments of the present invention andtherefore are to be regarded as schematic only. In particular, theelements shown in the drawings are not necessary to scale with eachother, and the placement of various elements in the drawings is chosento provide a clear understanding of the respective embodiment and is notto be construed as necessarily being a representation of the actualrelative locations of the various components in implementationsaccording to an embodiment of the invention.

It is to be noted that describing an embodiment comprising a pluralityof features is not to be construed as indicating that all these featuresare necessary for practicing the present invention. Instead, in otherembodiments, some features may be omitted, replaced by differentfeatures and/or additional features may be present.

The features of the various embodiments described herein may be combinedwith each other unless specifically noted otherwise.

The term “transmission channel” as used herein is intended to refer toany kind of transmission channel including wire-based transmissionchannels like a copper line or a pair of copper lines and wirelesstransmission channels.

The term “sub-channel” as used herein is intended to refer to asub-channel on a transmission channel, wherein on a single transmissionchannel a plurality of sub-channels may be present. For example, in DSLcommunication on a single wireline data is transmitted on a plurality ofcarriers having different frequencies, these carriers also beingreferred to as “tones”. Such carriers or tones are examples forsub-channels for the case of DSL data transmission. Another example fora sub-channel is a logic channel which may be used for transmittingspecific kinds of data, for example control information, wherein such alogic channel may use one or more of the above-mentioned carriers ortones in DSL communication.

Turning now to the Figures, in a communication system shown in FIG. 1, acommunication device 10 communicates with communication devices 16, 17,18 and 19 via respective transmission channels 12, 13, 14 and 15. Whilein FIG. 1 four communication devices 16, 17, 18 and 19 are shown, inother embodiments any suitable other number of communication devices mayalso be provided.

In an embodiment, the communication via transmission channels 12, 13, 14and 15 is a bidirectional communication. In such an embodiment,communication device 10 may comprise a transceiver for each of thetransmission channels 12, 13, 14 and 15, and each communication device16, 17, 18 and 19 also may comprise a transceiver. In anotherembodiment, all or some of transmission channels 12, 13, 14 and 15 maybe unidirectional transmission channels. In another embodiment, all orsome of the communication devices 16, 17, 18, 19 might be co-located.

In the embodiment of FIG. 1, couplings between the transmission channels12-15 may cause so-called far-end crosstalk (FEXT), for example if someor all of the transmission channels are wirelines running close to eachother. Through at least partial joint processing of the signalstransmitted from communication device 10 to communication device 16, 17,18 and 19 and/or through at least partial joint processing of signalsreceived from communication devices 16, 17, 18 and 19 at communicationdevice 10 in a crosstalk reduction unit 11, the influence of suchcrosstalk may be reduced. As already mentioned, the joint processing forcrosstalk reduction is also referred to as vectoring, and thetransmission channels which are subjected to such a crosstalk reductionare also referred to as vectored group.

In the following, the transmission direction from communication device10 to communication devices 16, 17, 18 and 19 will be referred to asdownstream direction, and the opposite transmission direction fromcommunication devices 16, 17, 18 and 19 to communication device 10 willbe referred to as upstream direction. Reduction of crosstalk in thedownstream direction is also referred to as crosstalk precompensationsince the signals transmitted are modified before transmission, i.e.before the actual crosstalk occurs, whereas the reduction of crosstalkin the upstream direction is also referred to as crosstalk cancellationas here through joint processing in crosstalk reduction unit 11 thecrosstalk is reduced or cancelled after it has occurred.

In embodiments, crosstalk cancellation may for example be performed bycalculating received signals for each transmission channel depending ona linear combination of all received signals on all transmissionchannels of the vectored group, and crosstalk precompensation may beperformed by calculating signals to be transmitted via each transmissionchannel depending on a linear combination of signals to be transmittedon all transmission channels. However, other calculation methods, forexample non-linear calculations, are also possible.

In order to perform such a crosstalk reduction, i.e. the vectoring, thecrosstalk reduction unit 11 has to be “trained”, i.e. the crosstalkreduction unit 11 needs information regarding the actual crosstalkoccurring between the transmission channels in the vectored group. Thismay for example be achieved by transmitting predetermined trainingsignals, for example pilot signals, via the transmission channels andanalyzing the received signals to determine the crosstalk. Inembodiments, data transmission via the transmission channels comprisesthe transmission of pilot signals or symbols, wherein between the pilotsignals other data like payload data may be transmitted. In anembodiment, the pilot signals or modified pilot signals are used fortraining crosstalk reduction unit 11. In an embodiment, synchronizationsignals or synchronization symbols may be used as pilot signals.However, other training signals may also be used.

In an embodiment, some or all of the transmission channels 12-15 of FIG.1 comprise a plurality of sub-channels.

It should be noted that in some cases sub-channels used for thedownstream direction will be different from sub-channels used for theupstream direction. For example, in DSL communication the sub-channelsfor the downstream direction may use (a) different frequency range(s)than the sub-channels in the upstream direction.

In such embodiments, for training in the downstream directioncommunication device 10 may transmit the above-mentioned trainingsignals on some or all sub-channels of communication lines 12 to 15 tocommunication devices 16 to 19. Communication devices 16 to 19 thenreturn error signals indicative of a deviation between the receivedtraining signals and the sent training signals back to communicationdevice 10. Based on these error signals, crosstalk reduction unit 11calculates first crosstalk reduction parameters for the downstreamdirection, which may also be referred to as crosstalk precompensationparameters or crosstalk precompensation coefficients. The error signalsconstitute crosstalk information indicative of the crosstalk occurringbetween the sub-channels of the communication channels 12 to 15. Asimilar approach may be made for the upstream direction.

In an embodiment, only some of the transmission channels connected tocommunication device 10 are subjected to vectoring. This is alsoreferred to as partial vectoring. A reason may for example be thecomputational complexity involved. For example, if 1,000 transmissionchannels are coupled with communication device 10, performing a completevectoring of these 1,000 transmission channels may in some cases exceedthe computational capabilities of processors or other equipment used forvectoring involved. Therefore, in such cases partial vectoring with onlysome of the transmission channels may be performed.

In such an embodiment, a selection is made with which of the pluralityof transmission channels present the vectoring is to be performed. Itshould be noted that this selection need not be the same for upstreamand downstream direction. The transmission channels which are involvedin the vectoring are also referred to as vectored group in thefollowing.

In an embodiment, different transmission channels may lead to differentcommunication devices, for example to communication devices 16 to 19 ofFIG. 1, of customers having different contracts with a provider of acommunication service via for example communication device 10. Forexample, different maximum data rates may be agreed upon in indifferentcontracts. In other embodiments, other criteria may be used todistinguish between transmission channels.

Based on such criteria, in an embodiment weighting coefficients orweighting factors are provided for the plurality of transmissionchannels coupled with communication device 10. The weightingcoefficients in an embodiment reflect the relative “importance” of thetransmission channels. For example, a transmission channel with a higherintended maximum data rate may have a higher weighting coefficient thana transmission channel with a lower intended maximum data rate.

In an embodiment, the selection which transmission channels to includein the vectored group is made depending on these weighting coefficients.Additionally, the selection may be made depending on the strength of thecrosstalk between the transmission channels.

In the following, some of the above concepts will be further illustratedusing further embodiments. As an example for an environment forimplementing the present invention, these embodiments DSL (digitalsubscriber line) systems like VDSL (very high bit rate DSL) systems areused.

In DSL systems, generally wire-based communication lines are used astransmission channels, for example pairs of copper lines, and on eachcommunication lines data is modulated onto a plurality of carrier ortones, i.e. different frequency sub-channels. However, it is to be notedthat in other embodiments other kinds of communication systems includingwireless communication systems may be used.

A DSL communication systems 20 shown in FIG. 2 comprises a plurality ofso-called subscribers 25. The equipment of subscribers 25 is sometimesalso referred to as customers premises equipment and may for examplecomprise DSL modems for connecting personal computers or other equipmentwith a so-called central office of a service provider.

In the Communication system 20 of FIG. 2, each of the subscribers 25 iscoupled to a central office equipment 26 via a respective transmissionchannel in the form of a communication line 23. In the example of FIG.2, the central office equipment 26 is a DSLAM (digital subscriber lineaccess multiplexer). In the embodiment of FIG. 2, DSLAM 26 comprises acrosstalk reduction and transmit power management unit 21 which controlsvectoring or partial vectoring of communication lines 23 by crosstalkcancellation or crosstalk precompensation (so-called Spectrum ManagementLevel 3) and to perform a power management, i.e. to adjust the transmitpower used by a different ones of communication lines 23 (SpectrumManagement Level 2).

The DSLAM 26 comprises a plurality of transmission ports, each coupledto a respective communication line 23. It should be noted while in theembodiment of FIG. 2 crosstalk reduction and transmit power managementunit 21 is located within DSLAM 26, in other embodiments unit 21 may bea unit external to DSLAM 26.

In the embodiment shown, the communication lines are bundled in a singlecable binder 22. Such bundling in a cable binder generally increases thelikelihood of crosstalk, for example far-end crosstalk, occurringbetween communication lines 23. However, in other embodiments some orall of communication lines 23 need not be in a common cable binder.

As illustrated in FIG. 2, the cable binder 22 may also compriseadditional communication lines 24 which are not used by the DSLcommunication system 20, for example analog telephone lines.

It should be noted that the communication lines 23 may have differentlengths, which is typical for a situation in which the individualsubscribers 25 are located at different positions. However, some or allof communication lines 23 may also have the same length.

The crosstalk reduction and transmit power management unit 21 maycontrol crosstalk reduction and the transmit power on the individuallines both for the downstream direction, i.e., the transmissiondirection from DSLAM 26 to subscribers 25, and in the upstreamdirection, i.e., in the direction from subscribers 25 to DSLAM 26.

In order to illustrate this, in FIG. 3 some components of a centraloffice equipment 30 according to an embodiment are schematically shownin FIG. 3. Central office equipment 30 may for example be incorporatedin DSLAM 26 of FIG. 2.

Central office equipment 30 shows some components used for transmittingdata in the downstream direction over a plurality of communication lines45, 46, 47. While three communication lines are shown as an example inFIG. 3, any number of communication lines may be present, for example1,000 communication lines or more.

Via communication lines 45, 46 and 47 communication device 30 is linkedto subscribers 48, 49, 50, the subscriber end of the communication linesbeing generally labelled 51 in the FIG. 3.

In device 30 of the embodiment of FIG. 3, symbol mappers 31, 32 and 33map data, for example payload or training data, onto carrierconstellations, i.e., a plurality of carriers or tones each carrierhaving its own frequency range, or, in other words, a plurality ofsub-channels. Said data is to be transmitted via communication lines 45,46, 47 to the respective subscribers 48, 49 and 50. A crosstalkprecompensator 37 which is an example for crosstalk reduction circuitrymodifies some or all of these symbol mappings in order to precompensatecrosstalk occurring during the transmission via communication lines 45,46, 47. In particular, in the embodiment of FIG. 3 crosstalkprecompensator 37 performs a so-called partial vectoring, i.e., only forsome of the communication lines involved vectoring is performed. Forexample, as indicated by arrows within precompensator 37 of FIG. 3, inthe example illustrated a partial vectoring is performed forcommunication lines 45 and 46, i.e., communication lines 45 and 46 forma vectored group, while no vectoring is performed for communication line47. This is only to be seen as an example and criteria for choosingwhich communication lines to add to a vectored group and correspondingmethods which may be implemented in control 44 which controls whichcommunication lines are added to the vectored group will be explainedfurther below.

For example, for partial vectoring, the symbols output by the symbolmappers associated with the communication lines of the vectored group,in the example shown symbol mappers 31 and 32, may be seen as a vectorwhich is multiplied with a matrix comprising crosstalk precompensationcoefficients. In other words, the symbols output by precompensator 37are, for the communication lines of the vectored group, linearcombinations of the symbol input to precompensator 37 for the vectoredgroup, where the crosstalk precompensation coefficients are chosen suchthat the effect of crosstalk between the communication line of thevectored group is precompensated. With the above method, far-endcrosstalk between the vectored lines may be reduced or cancelledaltogether. In other embodiments, instead of simply selectingcommunication lines to be added to the vectored group, certain FEXTbranches may be added. A FEXT branch in this respect is a crosstalk pathfrom a first transmission channel to a second transmission channel. Forexample, in the system of FIG. 3 for partial vectoring only crosstalkfrom communication line 47 to communication line 46 and fromcommunication line 46 to communication line 45 may be cancelled, whileother FEXT branches like from communication line 47 to communicationline 45 are not added to the vectored group.

The carrier mappings modified by precompensator 37 are modulated ontothe above mentioned plurality of carriers for each communication line.Power regulators 41, 42, 43 adjust transmit power levels of theindividual carriers and therefore of the signal to be transmitted viacommunication lines 45, 46 and 47. The individual power levels of thecarriers are controlled by control circuit 44 in the embodiment of FIG.3. Power regulators 41, 42, 43 may e.g. be implemented in a digitalsignal processor which multiplies the mapped constellations with poweradjustment factors. Then, the signals are transferred to signals in thetime domain by inverse fast Fourier transformers 38, 39 and 40.Possibilities for such a control will be discussed later. It should benoted that while in the embodiment of FIG. 3 transmit power levels areadjusted individually for each carrier, in other embodiments, thetransmit power may be adjusted globally for all carriers of acommunication line, for example by controlling an amplifier (not shown)downstream of the respective transformer 38, 39, 40.

Generally, the number of bits which can be mapped onto each carrier insymbol mappers 31 to 33 depends on the signal to noise ratio (SNR) forthe various carriers, which in turn depends both on the power level withwhich the signals are transmitted and on the amount of crosstalk.Therefore, precompensating crosstalk in precompensator 37 and/orincreasing the transmission power used for the individual carrier mayincrease the number of bits which can be mapped onto the respectivecarrier of the respective communication lines and therefore increase thebit rate. On the other hand, increasing transmission power for some orall carriers on one line may increase the crosstalk from this line toother lines, thereby potentially lowering the possible bit rate for theother lines. The number of bits which are mapped onto the respectivecarriers in the embodiment of FIG. 3 is also controlled by controlcircuit 44.

In order for example to determine the crosstalk between the variouslines and/or to determine precompensation coefficients to be used byprecompensator 37, as already mentioned training sequences, i.e., knownsymbols, are sent via communication lines 45, 46 and 47, and an errorsignal e is sent back to control circuit 44 by subscribers 48, 49 and50, the error signal e being indicative of deviations between thetransmitted signals sent by device 30 via communication lines 45, 46, 47and the symbols received by subscribers 48, 49, 50.

Furthermore, as an input control circuit 44 receives a weighting vectorG indicating relative weights of the various communication lines. Theseweights may for example be depending on an intended or agreed upon bitrate for the communication lines.

As a simple example, for communication lines 45 and 46 a first bit ratemay be intended, and for communication line 47 a second bit rate being ahalf of the first bit rate may be intended. In such a case, theweighting vector G may be set to be equal to (1, 1, 0.5) to indicatethat communication line 47 has half the intended bit rate compared withcommunication lines 45 and 46.

Control circuit 44 then controls precompensator 37, amplifiers 41 to 43and also symbol mappers 31 to 33 depending on the weighting vector G andthe error signal e.

It should be noted that in FIG. 3 only some components of communicationdevice 30 are depicted, and subscribers 48, 49 and 50 are depicted assimple blocks in order to simplify the explanations by showing onlyelements which are relevant for the understanding of the respectiveembodiment. Further components which are conventionally found in xDSLtransmission systems may additionally be present, like encoders forReed-Solomon coding or Tomlinson coding, serial/parallel andparallel/serial converters, and elements at subscribers 48, 49, 50 likefast Fourier transformers, filters, frequency equalizers or slicers.

Before examples for the operation of control circuit 44 will bedescribed in more detail first an embodiment of a corresponding systemand devices for operation in the upstream direction will be explainedwith reference to FIG. 4.

In the embodiment of FIG. 4, subscribers on a subscriber side 100transmit signals via communication lines 111, 112, 113 to central officeequipment 101 which may, for example, be a DSLAM like DSLAM 26 of FIG.2. It should be noted that for bidirectional communication systems, forexample communication lines 111, 112 and 113 of the embodiment of FIG.4, may be physically identical to communication lines 45, 46 and 47 ofthe embodiment of FIG. 3. In the embodiment of FIG. 4, for eachsubscriber a symbol mapper 102, 103 and 104, an inverse fast Fouriertransformer 105 and 106 and 107 and power regulators 108, 109 and 110,respectively, perform substantially the same functions and symbolmappers 31, 32, 33, inverse fast Fourier transformer 38, 39 and 40 andpower regulators 41, 42 and 43 in communication device 30 of theembodiment of FIG. 3, with the exception that no crosstalkprecompensator is provided. In other words, for each communication linea respective symbol mapper 102, 103 and 104 maps data onto a pluralityof carriers, the data is then modulated onto the carriers. The transmitpower of the individual carriers is adjusted via power regulators 108,109 and 110. The data is further transformed into the time domain byinverse fast Fourier transformers 105, 106 and 107 and transmitted viacommunication lines 111, 112 and 113. In case of bidirectionalcommunication, the carriers, i.e., frequency ranges, used for theupstream direction may be different from the carriers used for thedownstream direction. Similar to the downstream direction, the transmitpower may additionally or alternatively be adjusted for all carriers ofa communication line e.g. via an amplifier downstream of transformers105, 106, 107.

Power regulators 108, 109 and 110 are controlled by a control circuit121. Control circuit 121 in the embodiment of FIG. 4 is located incommunication device 101 on the central office side and may send controlsignals to amplifiers 108, 109 and 110 for example via a special logicchannel on communication lines 111, 112 and 113, for example an overheadchannel.

In communication device 101, received data is transferred to thefrequency domain via fast Fourier transformers 114, 115 and 116 and thenfed to a crosstalk canceller 117 which in the embodiment of FIG. 4 isconfigured to perform a partial vectoring for some of communicationlines 111, 112, 113. For example, in the situation shown in FIG. 4communication lines 112, 113 form a vectored group and symbols output bycrosstalk canceller 101 for these two lines are a linear combination ofsymbols received via these lines, similar to what has explained forcrosstalk precompensator 37. For example, the received symbols for thevectored group may be written as a vector and multiplied with a matrixcomprising crosstalk cancellation coefficients. The crosstalkcancellation in crosstalk canceller 101 is controlled by control circuit121, which for example controls which communication lines form thevectored group. As has been explained with respect to FIG. 3, also inthe embodiment of FIG. 4 instead of adding communication lines to thevectored group, selected FEXT branches may be added.

The symbols output by crosstalk canceller 101 are further processed inreceive circuitry 118, 119, 120 which may for example comprise frequencyequalizers or slicers. Similar to FIG. 3 it should be noted that both onthe subscriber line 100 and in communication device 101, furtherelements, for example elements conventionally found in DSL communicationsystems, may be present which, in order to provide a more conciseexplanation of features of the embodiments shown, are omitted in FIG. 4.

Similar to what was described with respect to the embodiment of FIG. 3,for determining crosstalk cancellation coefficients and/or fordetermining which lines to incorporate in the vectored group trainingsymbols are sent from subscriber side 100 to communication device 101,and an error signal e is generated and fed to control circuit 121. Alsoa weighting vector G which may be identical, but also may be differentto weighting vector G of FIG. 3, is fed to control circuit 121.

It should be noted that in a bidirectional communication systems controlcircuit 44 of FIG. 3 and control circuit 121 of FIG. 4 may beimplemented in a common circuit, for example in a common processor.Likewise, crosstalk precompensator 37 and crosstalk canceller 101 may beimplemented in the same circuit, which may comprise a processor, like adigital signal processor, which may, but need not be, the same processoras the one implementing control circuits 44 and 121. It should also benoted that in case of bidirectional communication, the elements andfeatures of communication devices 30, 101 may be implemented in a singlecommunication device, for example a single DSLAM.

Furthermore, while in the embodiments of FIGS. 3 and 4, control circuits44, 121 are shown as part of communication devices 30, 101, the controlcircuits may also be a further unit external to communication devices30, 101.

In the following, the operation of control circuits 44, 121 will bedescribed in more detail. In some embodiments, the operation isbasically the same both for the downstream and upstream direction andtherefore will be explained only once in the following. In other words,the following description applies to embodiments of the downstreamdirection and embodiments of the upstream direction, and also tobidirectional embodiments. With respect to bidirectional embodiments, itshould be noted that the choice for example which communication lines toincorporate into the vectored group may be made separately for the twocommunication directions, i.e., upstream and downstream, but may also bemade jointly, such that the same communication lines are part of thevectored groups in upstream and downstream direction. Also, thetransmission power of amplifiers used may be adjusted separately forupstream and downstream direction, but also may be adjusted jointly.

In the following, methods according to embodiments of the presentinvention will be described which methods may be for example beimplemented in crosstalk reduction and transmit power management unit 11of FIG. 1, in crosstalk reduction and transmit power management unit 21of FIG. 2 or in control circuit 44, 121 of FIGS. 3 and 4, but which mayalso be implemented separately therefrom.

According to an embodiment of the present invention, the choice whichcommunication lines or, more generally, which transmission channels orFEXT branches are to be incorporated into a vectored group is made basedon the strength of crosstalk between the transmission channels and basedon weighting coefficients or weighting factors attributed to thecommunication lines. In an embodiment which will be discussed next, aDSL system with M communication line is used as an example, wherein oneach communication line a number of carriers or frequency channels areused. The amount of data which may be transmitted over each carrier,i.e., the number of bits which may be loaded on a symbol of a carrier,depends on the signal to noise ratio (SNR) of this carrier. The signalto noise ratio (SNR(k))_(i) for a carrier k of a communication line iaccording to an embodiment is given by

$\begin{matrix}{{\left( {{SNR}(k)} \right)_{i} = \frac{{{\langle{u(k)}_{i}^{2}\rangle} \cdot {{H_{i}(k)}}^{2}}}{{\sum\limits_{\underset{j \neq i}{j = 1}}^{M}{{\langle{u(k)}_{j}^{2}\rangle} \cdot {{{Fext}_{ji}(k)}}^{2}}} + {\langle{r(k)}_{i}^{2}\rangle}}}{or}} & \left( {1a} \right) \\{\left( {{SNR}(k)} \right)_{i} = \frac{\langle{u(k)}_{i}^{2}\rangle}{{\sum\limits_{\underset{j \neq i}{j = 1}}^{M}{{\langle{u(k)}_{j}^{2}\rangle} \cdot \frac{{{{Fext}_{ji}(k)}}^{2}}{{{H_{i}(k)}}^{2}}}} + {\langle{r(k)}_{i}^{2}\rangle}}} & \left( {1b} \right)\end{matrix}$

In these equations, Fext_(ji)(k) describes the far end crosstalk (FEXT)coupling from line j to line i, H_(i)(k) describes the so-called linetransfer function of line i and <u(k)_(i) ²> and <r(k)_(i) ²> representthe variances of transmit symbols u(k)_(i) and of noise r(k)_(i) coupledinto line i, respectively.

As can be seen, reducing the crosstalk coupling, for example byvectoring or by adjusting transmit powers, increases the signal to noiseratio and therefore the possible throughput on a carrier andconsequently also on a corresponding line.

As mentioned above, according to some embodiments of the presentinvention, the choice which communication lines or

FEXT branches to add to the vectored group, i.e., between which lines tocancel crosstalk by crosstalk precompensation in the downstreamdirection or crosstalk cancellation in the upstream direction, is madebased on the strength of crosstalk between the lines. The far endcrosstalk power from a line j to line i may be written as

$\begin{matrix}{{{P\_ fext}_{ji}(k)} = {{\langle u^{2}\rangle} \cdot {\sum\limits_{k}\frac{{{{Fext}_{ji}(k)}}^{2}}{{{H_{i}(k)}}^{2}}}}} & (2)\end{matrix}$

Furthermore, as mentioned in large transmission systems with a pluralityof lines as mentioned previously not all subscribers may have a contractwhich guarantees them the maximum possible data rate. For example, basedon customer needs or charging tariffs, communication lines can bedivided into different groups characterized by different target bitrates, for example a high target bit rate and a low target bit rate. Insuch an embodiment, it may happen that even given the crosstalk, forlines belonging to a group with a low target bit rate this bit rate maybe achieved without any crosstalk reduction. Therefore, in such a case,such lines need not be added to the vectored group, i.e., crosstalk tothese lines need not be reduced through crosstalk precompensation orcrosstalk cancellation.

In an embodiment, to take different target bit rates into account, aweighting vector G is defined which describes the relative target bitrate of the different lines. The component of the vector G are denotedwith g_(i), g_(i) being the weighting factor or weighting coefficientattributed to line i. As an example, if there are two groups, one with ahigh target bit rate and one with a low target bit rate, and the low bitrate is half the high bit rate, g_(i) is set to 1 if line i belongs tothe group with a high target bit rate and is set to 0.5 when line ibelongs to the group with the low target bit rate.

To determine which FEXT branches to be added to the vectored group,according to an embodiment a weighted FEXT power matrix is calculatedaccording to

P_weight_(ji)(k)=P_fext_(ji)(k)·g _(j) ^(α) ·g _(i) ^(β)  (3)

wherein α and β are coefficients which will be described below. Thecomponents P_weight_(ji)(k) of this matrix describe the FEXT powercoming from line j and coupled into line i weighted by the weightingcoefficients.

g_(j) ^(α) relates to line j and g_(i) ^(β) relates to line i. α and βdetermine how strong the influence of the respective weighting factorg_(j) and g_(i) is.

If the relative target bit rates are 0.5 and 1 as in the above example,then according to an embodiment the low bit rate group uses a lowertransmit power, for example 3 db lower power corresponding to α=1 thanthe high bit rate group, such that α may be set to 1. However, othervalues may also be used. On the other hand, crosstalk cancellation orprecompensation to lines belonging to the low bit rate group in thiscase for example needs only to be applied in case of very strongcrosstalk, such that β may be selected to be greater than 1, for exampleβ=8. It should be noted that α and β are not limited to integer values.Through the selection of a high value of β, the elementsP_weight_(ji)(k) for line i belonging to the group with low bit rate aremade considerably smaller.

Based on the matrix elements determined according to equation (3), theFEXT branches or communication lines which are to be added to thevectored groups are selected. For example, for each line i, the matrixelements P_weight_(ji)(k) for all carriers k may be summed up, and ofall M(M−1) FEXT branches L branches with the highest results may beselected, L being the number of FEXT branches which can be incorporatedin the vectored group. L as explained previously may, for example, belimited due to the computation power available. In other embodiments, aspecific carrier k may be selected, and the L branches which have thelargest values P_weight_(ji)(k) for this carrier may be added to thevectored group. In still other embodiments, a number of carriers k, forexample carriers k having lower frequencies, carriers k having higherfrequencies or a number of carriers k being evenly distributed over thewhole spectral range used may be added and the result be used for theselection.

After selecting the FEXT branches or communication lines for partialvectoring, i.e., crosstalk cancellation or crosstalk precompensation,optionally, but not necessarily, spectrum balancing may be applied. Thespectrum balancing may use the weighting vector G defined above in orderto adjust the transmit power. Usable methods are, for example, describedin co-pending US application “Method of transmission power control andcommunication device”, U.S. Ser. No. 11/950,283, the whole content ofwhich is incorporated by reference herein for all purposes.

In the following, an example for spectrum balancing which may be appliedin embodiments of the present invention will be described in some moredetail.

In general, as mentioned previously, the maximum bit rates are afunction of the transmission powers of the individual transmissionlines, i.e.

$\begin{matrix}{{B = {f(P)}},{wherein}} & (4) \\{B = \begin{pmatrix}B_{1} \\\vdots \\B_{M}\end{pmatrix}} & (5)\end{matrix}$

is a bit rate vector,

$\begin{matrix}{P = \begin{pmatrix}P_{1} \\\vdots \\P_{M}\end{pmatrix}} & (6)\end{matrix}$

is a transmission power vector

M denotes the number of transmission lines and f is a function. Eachcomponent B_(i) of the bit rate vector corresponds to the maximum bitrate of the respective individual transmission line i, and eachcomponent P_(i) of the transmission power vector corresponds to thetransmission power of the respective individual transmission line i.

According to an embodiment of the invention, a method of transmissionpower adjustment starts with the already defined weighting vector

$\begin{matrix}{G = \begin{pmatrix}g_{1} \\\vdots \\g_{M}\end{pmatrix}} & (7)\end{matrix}$

According to the embodiment, the transmission powers of the individualtransmission lines are then iteratively calculated in such a way thatthe actual or effective relative bit rate distribution conforms to orapproximates the nominal relative bit rate distribution.

In the following,

$\begin{matrix}{P_{k} = \begin{pmatrix}P_{1,k} \\\vdots \\P_{M,k}\end{pmatrix}} & (8)\end{matrix}$

denotes the transmission power vector in the k-th iteration step.

From the components of the transmission power vector in the k-thiteration step, the transmission power vector in the (k+1)-th iterationstep is calculated according to

$\begin{matrix}{{P_{k + 1} = \begin{pmatrix}{P_{1,k} \cdot K_{1,k}} \\\vdots \\{P_{M,k} \cdot K_{M,k}}\end{pmatrix}}{wherein}} & (9) \\{K_{k} = {\begin{pmatrix}K_{1,k} \\\vdots \\K_{M,k}\end{pmatrix} = \begin{pmatrix}s^{{- \Delta}\; {B_{1,k} \cdot {\alpha {(k)}}}} \\\vdots \\s^{{- \Delta}\; {B_{M,k} \cdot {\alpha {(k)}}}}\end{pmatrix}}} & (10)\end{matrix}$

is a scaling vector calculated on the basis of the bit rate vector

$\begin{matrix}{B_{k} = \begin{pmatrix}B_{1,k} \\\vdots \\B_{M,k}\end{pmatrix}} & (11)\end{matrix}$

in the k-th iteration step.

From the bit rate vector and the weight vector, a weighted bit ratevector

$\begin{matrix}{{Bg}_{k} = {\begin{pmatrix}{Bg}_{1,k} \\\vdots \\{Bg}_{M,k}\end{pmatrix} = \begin{pmatrix}{B_{1,k}\text{/}G_{1}} \\\vdots \\{B_{M,k}\text{/}G_{M}}\end{pmatrix}}} & (12)\end{matrix}$

is calculated. From the weighted bit rate vector, a difference vector iscalculated according to

$\begin{matrix}{{\Delta \; B_{k}} = {\begin{pmatrix}{\Delta \; B_{1,k}} \\\vdots \\{\Delta \; B_{M,k}}\end{pmatrix} = {\begin{pmatrix}{{Bg}_{1,k} - {\min \left\{ {{Bg}_{1,k},\ldots,{Bg}_{M,k}} \right\}}} \\\vdots \\{{Bg}_{M,k} - {\min \left\{ {{Bg}_{1,k},\ldots,{Bg}_{M,k}} \right\}}}\end{pmatrix}.}}} & (13)\end{matrix}$

The components of the difference vector are used in Equation (7) forcalculating the scaling vector. In Equation (7) s and α (k) are selectedto be larger than one.

According to an embodiment, s=10. Further, α(k) may be varied dependingon the iteration step, i.e. as a function of k. By this means,convergence speed and precision of the iteration process can beadjusted. In other embodiments, other values of may be selected, andα(k) may have the same value irrespective of the iteration step, e.g.α(k)=1.

Summarizing the above Equations (9)-(13), the components of thetransmission power vector in the (k+1)-th iteration step may thus becalculated according to

P _(i,k+1) =P _(i,k) ·s ^(−(Bg) ^(i,k) ^(−min{Bg) ^(1,k) ^(, . . . , Bg)^(M,k) ^(})α(k)).  (14)

Next, the calculated transmission power vector is subjected to a scalingoperation according to

$\begin{matrix}{P_{k}\mspace{14mu} \text{:=}\mspace{14mu} {P_{k} \cdot {\frac{P_{\max}}{\max \left\{ {P_{1,k},\ldots,P_{M,k}} \right\}}.}}} & (15)\end{matrix}$

In other words, the components of the scaled transmission power vectormay be calculated according to

$\begin{matrix}{P_{i,k}\mspace{14mu} \text{:=}\mspace{14mu} {P_{i,k} \cdot {\frac{P_{\max}}{\max \left\{ {P_{0,k},\ldots,P_{M,k}} \right\}}.}}} & (16)\end{matrix}$

That is to say, the components of the transmission power vector in thek-th iteration step are scaled in such a way that the largest componentof the transmission power vector corresponds to a maximum admissibletransmission power P_(max) of a transmission line.

On the basis of the scaled transmission power vector, the transmissionpower control system, for example control circuit 44 of FIG. 3 orcontrol circuit 121 of FIG. 4, calculates transmission power spectraldensities of the individual transmission lines and the correspondingmaximum achievable bit rate, i.e. the bit rate vector of the nextiteration step. For this purpose, an optimization algorithm, such as thewaterfilling algorithm, may be applied. The waterfilling algorithm andother types of suitable optimization algorithms will be explained below.This process may be iteratively repeated until the deviation of theresulting relative bit rate distribution from the nominal relative bitrate distribution, i.e. from the weight vector, is below a predefinedthreshold or a maximum number of iteration steps is reached.

A bit number R_(j) of the j-th frequency sub-channel, i.e., carrier, ofa transmission channel can be written as

$\begin{matrix}{{R_{j} = {\log_{2}\left( {1 + \frac{\left( {S\text{/}N} \right)_{j}}{\Gamma}} \right)}},} & (17)\end{matrix}$

(S/N)_(j) being the signal-to-noise ratio of the j-th frequencysub-channel at the receiver and Γ being the signal-to-noise gapparameter. The signal-to-noise gap parameter is selected to be at least1 dB, typically in a range from 5 dB to 20 dB.

In some embodiments, the signal-to-noise gap parameter may be frequencydependent, i.e. a function of the sub-channel index j.

The total bit rate of the transmission channel can be obtained bysumming the bit numbers of the frequency sub-channels and bymultiplication with the symbol frequency according to

$\begin{matrix}{{B = {f_{T} \cdot {\sum\limits_{j}\; R_{j}}}},} & (18)\end{matrix}$

f_(T) being the symbol frequency.

The signal-to-noise ratio at the receiver can be represented by

$\begin{matrix}{\left( {S\text{/}N} \right)_{j} = {\frac{\left. {p_{j} \cdot} \middle| H_{j} \right|^{2}}{\Gamma \cdot \sigma_{j}^{2}}.}} & (19)\end{matrix}$

Accordingly, a total bit rate function of the transmission channel canbe expressed as

$\begin{matrix}{{B = {f_{T} \cdot {\sum\limits_{j}\; {\log_{2}\left( {1 + \frac{\left. {p_{j} \cdot} \middle| H_{j} \right|^{2}}{\Gamma \cdot \sigma_{j}^{2}}} \right)}}}},} & (20)\end{matrix}$

In the optimization algorithm, the total bit rate function is maximized.This is accomplished with the additional condition that a maximum valueof the transmission power is defined according to

$\begin{matrix}{P_{\max} = {\sum\limits_{j}\; {p_{j}.}}} & (21)\end{matrix}$

Maximizing the bit rate function as defined in Equation (20) with theadditional condition of Equation (21) results in the above-mentionedwaterfilling algorithm. The mathematical details of solving theoptimization problem are known in the art and will not be furtherexplained herein.

According to further embodiments of the invention, differentoptimization algorithms than the waterfilling algorithm may be used foradjusting the transmission power spectral densities.

According to one embodiment of the invention, an optimization algorithmis used in which the bit rate function of equation (20) is simplified to

$\begin{matrix}{B = {f_{T} \cdot {\sum\limits_{j}\; {{\log_{2}\left( \frac{\left. {p_{j} \cdot} \middle| H_{j} \right|^{2}}{\Gamma \cdot \sigma_{j}^{2}} \right)}.}}}} & (22)\end{matrix}$

Maximizing the total bit rate function as given by equation (22) withthe additional condition of equation (21) results in an optimizationalgorithm may be referred to as “simplified waterfilling algorithm”.

The result of the simplified waterfilling algorithm is a piecewiseconstant transmission power spectral density, i.e. all values of p_(j)are the same or zero. The values of p_(j) and the number of usablefrequency sub-channels are selected in such a way that on the one hand

$\begin{matrix}{\frac{\left. {p_{j} \cdot} \middle| H_{j} \right|^{2}}{\sigma_{j}^{2}} \geqq \Gamma} & (33)\end{matrix}$

and on the other hand

$\begin{matrix}{{p_{j} = \frac{P_{\max}}{N}},} & (34)\end{matrix}$

N denoting the number of usable frequency sub-channels.

Again, it is refrained from discussing mathematical details of solvingthe simplified optimization problem, as these are known in the art.

As compared to the waterfilling algorithm, the simplified waterfillingalgorithm significantly reduces the computational effort when adjustingthe transmission power spectral densities. This is specificallyadvantageous in a method of transmission power control of multipletransmission channels, e.g. as explained above. In other embodiments,the simplified waterfilling algorithm may also be applied to a singletransmission channel or in other methods of adjusting the transmissionpowers of multiple transmission channels.

In the above embodiments, a weighting vector G was used which reflectsrelative desired bit rates for various transmission channels, forexample various communication lines in xDSL systems. In otherembodiments, some elements of such a weighting vector may not be fixedlyset in advance, but only be determined e.g. during the initialization ofthe system. For example, transmission channels may be assigned avariable weight. In such an embodiment, the variable weight may beadjusted such that when intended bit rates for transmission channelshaving fixed weights are reached, the bit rates for transmissionchannels with variable weights are maximized. An example for such anembodiment will now be explained with reference to FIG. 5.

In the embodiment of FIG. 5, a plurality of transmission channels isdivided into three groups, a first group with a first intended targetbit rate, a second group with a second intended target bit ratedifferent from the first target bit rate, and a third group oftransmission channels which has no specified target bit rate, but thetransmission channels of which should eventually have at leastapproximately the same bit rate.

At 140, an initial weighting vector G with weighting factors g_(i) foreach transmission channel is defined. g_(i) may be either a, b or x,wherein a is the weighting factor for the transmission channel of thefirst group, b is the weighing factor for the transmission channels ofthe second group and x is the weighting factor for the transmissionchannels of the third group. x is initially set to 0 and a and b arechosen such that the ratio a/b corresponds to the ratio between thefirst bit rate and the second bit rate. For example, if the first bitrate is twice the second bit rate, a may be set to 1 and b may be set to0.5.

At 141, the values P_weight_(ij) are determined as defined by equation(3).

At 142, the FEXT branches to be added to a vectored group are selectedbased on the values calculated at 141, for example as already described.As an example, for M transmission channels M(M−1) FEXT branches exist,and of these M(M−1) branches L branches may be added to the vectoredgroup in a particular implementation.

At 143, a spectrum balancing, i.e., an adjustment of transmission powersfor the various transmission channels, for example as described above,is performed. It is to be noted that the spectrum balancing at 143 isoptional, and may be omitted in other embodiments.

With the crosstalk reduction determined at 142 and the spectrumbalancing performed at 143, at 144 the bit rate achieved for one of thegroups, for example the group with the highest bit rate (labeled group_1in FIG. 6) is determined, and from this value the target bit rate, i.e.,the intended bit rate, for this group is subtracted to calculate a bitrate difference ΔB. In other embodiments, such a bit rate difference maybe calculated for more than one group. At 145, it is checked if theabsolute value of ΔB is smaller than a predetermined tolerance value.The predetermined tolerance value indicates how exact the target bitrate should be reached. For example, the tolerance value may be 10% ofthe target bit rate, 5% of the target bit rate or 1% of the target bitrate, although other values are equally possible.

If the absolute value of ΔB is smaller than the tolerance value, at 146the procedure is ended since this indicates that the bit rates have beenadjusted with sufficient accuracy. If this is not the case, at 147 it ischecked if ΔB is smaller than 0. ΔB<0 means that the actual bit rate issmaller than the target bit rate. If this is the case, at 148 theweighting value x is reduced by a value Δx which is depending on ΔB. Forexample, Δx may correspond to ΔB multiplied by a predetermined factor.

In an embodiment, at a first iteration step of the method shown in FIG.6, i.e., in the first run of the operation described so far, the methodmay be terminated if ΔB<0 since in this case, i.e., in the firstiteration, x=0 and therefore cannot be further reduced. This indicatesthat with the intended distribution of the transmission channels to thefirst group, second group and third group the target bit rate(s) may notbe reached. In an embodiment, in such a case the target bit rate(s) maybe reduced and the method shown may be started anew at 140, and/or sometransmission channels may be assigned to a different group correspondingto a lower bit rate. For example, if the first bit rate is twice thesecond bit rate, some transmission channels may be moved from the firstgroup to the second group, and then the method may be started again at140.

If, on the other hand, at 147 it turns out that ΔB>0, this means thatthe actual bit rate is higher than the target bit rate. This, in turn,means that the actual bit rate of the first group and the second groupmay be lowered while still obtaining the target bit rate, and the bitrate of the third group may be increased. Consequently, in this case at149 x is increased by a value Δx depending on ΔB, for examplecorresponding to ΔB multiplied by a predetermined factor.

Corresponding to the new values for x calculated at 148 or 149, at 150 anew weighting vector is defined with the weights for the transmissionchannels of the first group set to the new value of x, and then themethod is continued again at 141 with the new values.

With a method as described with respect to FIG. 6, bit rates of thefirst group and the second group may be set to the intended values, andthe bit rates of the transmission channels of the first group may be setto a third uniform value. It should be noted that the use of threegroups of transmission channels in the above explanation is only anexample and any arbitrary number of groups may be used. For example,five groups may be used, where three of the groups have fixed assignedweighting factors, for example 1, 0.7 and 0.3, and the two other groupsmay have variable weighting factors having a predetermined relationshipwith each other, for example 2x and x. Any other desired values are alsopossible. It is to be noted that the embodiment discussed with referenceto FIG. 5 may be employed both for the upstream direction and thedownstream direction, for example may be implemented in control circuit44 of the embodiment of FIG. 3 or control circuit 121 of the embodimentof FIG. 4.

In the following, the operation of the embodiment discussed withreference to FIG. 5 will be further illustrated using simulationresults. As an example for a communication system, a DSL system as forexample shown in the embodiments of FIGS. 3 and 4 is used. Inparticular, as a central office equipment for this example a VDSL-2 linecard supporting 48 communication line which are all contained in onecable binder, wherein no other lines are present in this cable linewhich may disturb these lines, is used. A white noise signal of −140dBm/Hz is assumed to disturb all lines. The lengths of the communicationlines are equally distributed in the range of 400 to 450 m. The cablebinder for this simulation is assumed to be of the type AWG26, and theso-called MIMO crosstalk channel model is used for modeling far endcrosstalk. The maximum used frequency range is 12 MHz for allcommunication lines.

In the simulation example, partial crosstalk cancellation or partialcrosstalk precompensation is assumed for basically eliminating crosstalkof 16×15=240 FEXT branches, i.e., for 16 lines added to the vectoredgroup, out of 48×47=2256 branches. For the stimulation example, as inthe embodiment of FIG. 5 three groups of communication lines arere-defined. For the first group, a target bit rate of 65 Megabit persecond (MBit/s) for the downstream direction and 35 Mbit/s for theupstream direction is selected. For the second group, a target bit rateof 32.5 Mbit/s for the downstream direction and 17.5 Mbit/s for theupstream direction is defined. In other words, the target bit rate forthe second group is half the target bit rate for the first group both inthe downstream direction and in the upstream direction. For the thirdgroup no fixed target bit rate is defined, corresponding to a variableweighting factor as described with reference to FIG. 5, but allcommunication lines associated with the third group should get the samemaximum bit rate both in the upstream direction and in the downstreamdirection. This bit rate for the communication lines of the third groupshould be determined such that the target bit rates for the first groupand the second group are provided.

In the simulation example, 32 randomly selected communication lines areallocated to the first group, eight randomly selected communicationlines are allocated to the second group, and the remaining eightcommunication lines are associated with the third group. Therefore, asalso mentioned in an example with respect to FIG. 5 above, for thesimulation the components of the weighting factor G assigned to thefirst group are set to 1, the components assigned to the second groupare set to 0.5 and the components assigned to the third group are set toa variable x, which is set to 0 as an initial value.

With this system, methods as described with respect to FIG. 5 areperformed.

FIGS. 6 and 7 show simulation results in the downstream and upstreamdirection, respectively, with the method as discussed with respect toFIG. 5 applied wherein the spectrum balancing at 143 has been omitted.In other words, in FIGS. 6 and 7 the situation is shown where onlypartial crosstalk cancellation or precompensation, but no spectrumbalancing is applied.

In FIGS. 6 and 7, the achieved data rate in megabits per second isplotted over the line number. A curve 55 as a reference shows adistribution of maximum bit rate reached for the various lines with theuse of partial crosstalk cancellation, but without taking the groupinginto consideration, which for example would correspond to a weightingvector where all weighting factors are equal. As can be seen, in thiscase the maximum bit rates for all lines is in the same range, but nolines reach the target bit rate for the first group.

Curve 56 shows the maximum bit rates with partial crosstalk cancellationand the grouping taking into account, i.e., a weighting vector withthree weighting factors 1, 0.5 and x as a variable weight is used. Inthe graph, the lines are arranged according to their group, and withintheir groups are sorted according to their bit rate to make the graphseasier to comprehend. In other words, for curve 56 line 1 to 32 belongto the first group, lines 33 to 40 belong to the second group and lines41 to 48 belong to the third group. As can be seen, through the use ofgrouping the bit rates of the lines of the first group are increasedcompared with the situation without grouping, and all exceed 60 Mbit/s.

Similar to FIG. 6, in FIG. 7 a curve 60 shows the bit rates with partialcrosstalk cancellation, but without the grouping taken into account, andcurve 61 shows the maximum bit rates with partial crosstalkconcentration and grouping taking into account by using thecorresponding weighting vector. The arrangement of lines for curve 61corresponds to one explained for curve 56. Also in this case, throughthe use of a weighting vector higher bit rates can be obtained for thelines of the first group.

In FIG. 8 for the downstream direction, curve 56 of FIG. 6 is shownagain as a reference and in FIG. 9 for the upstream direction curve 61of FIG. 7 is shown again. Additionally, curves are shown which show thereachable bit rates with an embodiment which uses both partial crosstalkcancellation or precompensation and spectrum balancing taking thegrouping, i.e., the weighting vector, into account. In FIG. 8, curve 66shows the obtainable bit rates in such an embodiment, and in FIG. 9curve 71 shows the obtainable bit rates for such an embodiment. Thefirst 32 lines again are the lines of the first group, the next eightlines the lines of the second group and the last eight lines the linesof the third group. As can be seen, in the simulation example withpartial crosstalk precompensation or cancellation and spectrumbalancing, both the lines of the first group and the lines of the secondgroup are adjusted to their target bit rate, and the lines of the thirdgroup all have the same maximum bit rate. In the example shown, themaximum bit rate for the third group are 17.6 Mbit per second in thedownstream direction and 0.75 Mbit per second in the upstream direction.In an embodiment, if these bit rates for the third group are too low,for example are below a predetermined value, predefined network orsystem parameters like the distribution of lines to the first group andthe second group and/or the target bit rates for the first group and thesecond group may be changed.

In FIGS. 10 and 11, the transmit power for all users is shown. FIG. 10shows the transmit powers for the downstream direction, and FIG. 11shows the transmit powers for the upstream direction.

Curve 75 in FIG. 10 and curve 80 in FIG. 11 show a case where for alllines a maximum transmit power of 11.5 dBm is used. In this case, nocrosstalk reduction and no spectrum balancing is applied. Therefore,curve 75 and 80 may serve as reference curves.

Curve 77 in FIG. 10 and curve 82 in FIG. 11 show an example for transmitpowers with partial crosstalk cancellation or precompensation whentaking the grouping, i.e., the weighting vector, into account. Lines 1to 32 again belong to the first group, lines 33 to 40 to the secondgroup and lines 41 to 48 to the third group. In this case, only for thelines of the first group the maximum transmit power is used, and thelines of the second and third group have reduced transmit powers. Curves77 and 82 therefore correspond to the situation represented by curve 56and 61, respectively, in FIGS. 6 to 9.

Curve 76 in FIG. 10 and curve 81 in FIG. 11 show an example for transmitpowers with partial crosstalk precompensation or cancellation andspectrum balancing under consideration of the grouping. Curves 76 and 81therefore correspond to the situation of curves 66 and 71 of FIGS. 8 and9, respectively.

With spectrum balancing the overall transmit power may be reduced insuch an embodiment, thus reducing power consumption and non-lineardistortion.

In one implementation a communication device may include a plurality ofoutputs to couple with a plurality of communication lines, a pluralityof symbol mappers, each symbol mapper being associated with a respectiveoutput and configured to map data to be transmitted over the respectivecommunication lines to a data symbol, a crosstalk precompensatorconfigured to combine data symbols generated by a part of said symbolmappers to generate crosstalk precompensated data symbols for a part ofsaid plurality of communication lines associated with said part of saidsymbol mappers, and a control circuit configured to select said part ofcommunication lines depending on crosstalk strength between saidcommunication lines and weighting factors associated with said pluralityof communication lines.

The communication device according to the above wherein said controlcircuit is configured to select said part of communication lines ascommunication lines associated with crosstalk branches which have thehighest crosstalk strength weighted with weighting factors of thecommunication lines associated with the crosstalk branch.

The communication device according to the above wherein on each of saidplurality of communication lines data is transmitted on a plurality ofcarriers, wherein a weighted cross-talk strength P_weight_(ji)(k) for acarrier k of a crosstalk branch from a communication line j to acommunication line i is calculated according to

P_weight_(ji)(k)=P_fext_(ji)(k)·g _(j) ^(α) ·g _(i) ^(β)

wherein P_fext_(ji)(k) is the crosstalk power from communication line jto communication line i for carrier k, g_(j) is the weighting factor forline j, g_(i) is the weighting factor associated with line i and α and βare predetermined values, and wherein the control circuit is configuredto select said part of said communication lines depending on saidweighted crosstalk strengths.

The communication device according to the above wherein β>1.

The communication device according to the above wherein said controlcircuit is further configured to control an output power for each ofsaid communication lines depending of said weighting factors.

The communication device according to the above wherein said weightingfactors represent relative bit rates, and wherein said transmissionpowers are adjusted such that transmission rates obtained on saidplurality of communication lines match the relative bit rates.

In another implementation a method may include adjusting bit rates for aplurality of transmission channels based on weighting factors associatedwith the plurality of transmission channels, wherein said adjusting bitrates comprises a partial vectoring.

It should be noted that the above described embodiment and simulationexamples serve only for illustrating some possibilities for implementingthe present invention and are not to be construed as limiting. Forexample, the numerical values and number of groups given in thesimulation examples of FIGS. 6 to 11 serve only for illustrating somefeatures of some embodiments of the present invention. The scope of thepresent application is to be construed as limited only by the appendedclaims and equivalents thereof.

1. A communication device, comprising: communication circuitryconfigured to communicate via a plurality of transmission channels,wherein said communication circuitry comprises crosstalk reductioncircuitry to reduce crosstalk for a part of said plurality oftransmission channels by joint processing of data of said part of saidtransmission channels, wherein said part is selected from said pluralityof transmission channels depending on a grouping of said transmissionchannels into at least two groups.
 2. The communication device of claim1, wherein to each of said at least two groups a target bit rate isassigned, wherein the target bit rate for different groups differ. 3.The communication device of claim 2, wherein said target bit ratescomprises at least one fixed target bit rate and at least one variabletarget bit rate.
 4. The communication device of any one of claims 1-3,wherein said communication device is configured to select said partdepending on a weighting vector describing relative target bit rates ofthe plurality of transmission channels.
 5. The communication device ofdevice of claim 1, further comprising: transmission power controlcircuitry configured to control transmission power on said plurality oftransmission channels depending on said grouping.
 6. The communicationdevice of any one of claims 1-5, wherein said communication device isconfigured to select said part of said plurality of transmissionchannels depending on crosstalk strengths between said plurality oftransmission channels.
 7. The communication device of device of claim 1,wherein said plurality of transmission channels are a plurality of xDSLcommunication lines.
 8. The communication device of device of claim 1,wherein selecting said part comprises selecting far-end crosstalkbranches between said communication channels based on said grouping, andwherein reducing said crosstalk comprises reducing said crosstalk forsaid selected far-end crosstalk branches.
 9. A communication device,comprising: communication circuitry configured to communicate via aplurality of transmission channels, wherein said communication circuitrycomprises crosstalk reduction circuitry to reduce crosstalk for a partof far-end crosstalk branches between said plurality of transmissionchannels by joint processing of data corresponding to said part of saidfar-end crosstalk branches, wherein said part is selected depending on agrouping of said transmission channels into at least two groups.
 10. Acommunication device, comprising: a plurality of inputs to be coupledwith a respective plurality of communication lines, receive circuitryconfigured to obtain receive symbols based on data received via each ofsaid plurality of communication lines, a crosstalk canceller configuredto combine receive symbols received via a part of said communicationlines to generate crosstalk cancelled receive symbols, and a controlcircuit configured to select said part of said communication linedepending on a crosstalk strength between said communication lines andweight factors associated with said plurality of communication lines.11. The communication device of claim 10, wherein said control circuitis configured to select said part of communication lines ascommunication lines associated with crosstalk branches which have thehighest crosstalk strength weighted with weighting factors of thecommunication lines associated with the crosstalk branch.
 12. Thecommunication device of claim 11, wherein the communication device isconfigured to transmit on each of said plurality of communication linesdata on a plurality of carriers, wherein a weighted crosstalk strengthP_weight_(ji)(k) for a carrier k of a crosstalk branch from acommunication line j to a communication line i is calculated accordingtoP_weight_(ji)(k)=P_fext_(ji)(k)·g _(j) ^(α) ·g _(i) ^(β) whereinP_fext_(ji)(k) is the crosstalk power from communication line j tocommunication line i for carrier k, g_(j) is the weighting factor forline j, g_(i) is the weighting factor associated with line i and α and βare predetermined values, and wherein the control circuit is configuredto select said part of said communication lines depending on saidweighted crosstalk strengths.
 13. The communication device of claim 12,wherein β>1.
 14. The communication device of claim 10, wherein saidcontrol circuit is further configured to control transmit powers ofsignals received on said inputs depending of said weighting factors bycontrolling further communication devices connected to far ends of saidcommunication lines.
 15. The communication device of claim 10, whereinsaid weighting factors represent relative bit rates, and wherein saidtransmission powers are adjusted such that transmission rates obtainedon said plurality of communication lines match the relative bit rates.16. A method comprising: defining weighting factors for a plurality oftransmission channels, determining crosstalk strength between saidplurality of transmission channels, selecting a part of the plurality oftransmission channels depending on said crosstalk strength and dependingon said weighting factors for crosstalk reduction, and communicatingdata via said plurality of transmission channels using crosstalkreduction for said part of said plurality of transmission channels. 17.The method of claim 16, wherein said weighting factors indicate relativetarget bit rates for said plurality of communication channels.
 18. Themethod of claim 16, wherein said weighting factors comprise at least onefixed weighting factor and at least one variable weighting factor. 19.The method of claim 18, further comprising adjusting said variableweighting factor such that bit rate ratios defined by said at least onefixed weighting factor are fulfilled.
 20. The method of claim 16,further comprising performing a spectrum balancing for said plurality oftransmission channels depending on said weighting factors.
 21. Themethod of claim 16, wherein selecting said part comprises selectingfar-end crosstalk branches between said communication channels based onsaid grouping, and wherein said using of crosstalk reduction comprisesreducing said crosstalk for said selected far-end crosstalk branches.